Variable vane assembly

Abstract

A variable vane assembly, for example of stator vanes in a gas turbine engine, comprises vanes which can be turned together about their longitudinal axes by means of a unison ring which is turned by an actuator about the engine axis. The unison ring is coupled to the vanes by levers. The unison ring has varying stiffness along its circumference, increasing in the direction away from the drive point at which the actuator acts. The varying stiffness may be achieved by varying the radial thickness of the unison ring. The unison ring is thus able to resist ovalization so that the vanes move together.

1. A variable vane assembly comprising:
 a unison ring having a radial stiffness that varies in a circumferential direction thereof; 
 an array of variable vanes coupled to the unison ring for common displacement upon rotation of the unison ring about its central axis by means of a force applied at a first drive point and at a second drive point on the unison ring;
 a first actuator that is: i) configured to rotate the unison ring about its central axis, and ii) connected to the unison ring at the first drive point, the first drive point being positioned in a region of minimum stiffness on the unison ring; and 
 a second actuator that is: i) configured to rotate the unison ring about its central axis, and ii) connected to the unison ring at the second drive point, the second drive point being positioned in a region that is diametrically opposite to the region of minimum stiffness. 
 
 
     
     
       2. The variable vane assembly as claimed in  claim 1 , wherein
 the radial stiffness of the unison ring varies over at least 50% of a circumference of the unison ring. 
 
     
     
       3. The variable vane assembly as claimed in  claim 1 , wherein
 the radial stiffness varies progressively with distance from the first drive point and the second drive point. 
 
     
     
       4. The variable vane assembly as claimed in  claim 1 , wherein
 a radial dimension of a cross-section of the unison ring varies circumferentially to provide variation in the radial stiffness. 
 
     
     
       5. The variable vane assembly as claimed in  claim 1 , wherein
 the unison ring comprises a first member having a uniform cross-section and a second reinforcing member providing variation in the radial stiffness. 
 
     
     
       6. The variable vane assembly as claimed in  claim 5 , wherein
 the second reinforcing member has a cross-section which varies circumferentially. 
 
     
     
       7. A gas turbine engine comprising the variable vane assembly in accordance with  claim 1 .